The invention relates to an upper thread tension adjusting device for a sewing machine.
It is known that the selection or designation of the type of thread to be used and/or the type of fabric to be sewn will indicate the degree of thread tension to be subsequently adjusted as taught in U.S. Pat. No. 4,236,467 of the same assignee.
Until now, as shown in FIG. 6, thread tensions (a', b', c') of the sewing machines may be identical at one of the graduations of a display part, but may be different at the other graduations due to the possible discrepancies and deviations in precision of elements constituting the upper thread adjusting devices of the sewing machine. It has therefore been practically difficult to show the upper thread tension by the graduations.
The upper thread tension device of the sewing machine has the relationship between the thread tension and the indication of the strength of a spring provided for the thread tension as shown in FIG. 5. Each of the thread tension devices has individually different characteristics.
Conventionally, individual differences are regulated, as shown in FIG. 6, by handling an upper thread tension at a certain point.